1. Field of the Invention
The present invention generally relates to a Printing Circuit Board, and more particularly related to a PCB which is comprised of an insulator and a trace embedded therein, wherein both the upper surface of trace and a portion of the lower surface of trace exposed to the atmosphere for connecting external substance.
2. Description of the Related Art
Nowadays, both science and technology are developing rapidly, meanwhile, electrical devices are designed for thinner, wider application, lower cost, and better reliability, wherein in order to achieve above objects, the manufacturers keep on developing it.
FIG. 9 shows a conventional PCB 52 comprised of: an insulator 40 which is made of FR-4 or BT (Bismaleimide Triazine) or epoxy etc. having a blind via 44, an upper surface 41 and a lower surface 42; said blind via 44 is for accommodating a portion of trace 7A; a ball pad 7B which is made of metal such as copper for external connection (e.g. solder ball), said ball pad 7B having a side edge 7B3, an upper surface 7B1, and a lower surface 7B2, wherein both said side edge 7B3 and a portion of lower surface 7B2 of trace 7A encapsulated by said insulator 40, wherein another portion of lower surface 7B2 exposed to blind via 44 for being coupled with said trace 7A, said upper surface 7B1 of ball pad 7B exposed out of the lower surface 42 of insulator 40 for external connection (i.e. said upper surface 7B1 of ball pad 7B exposed to atmosphere for external connection); a trace 7A which is made of metal such as copper for being employed as a path of transmitting electrical current, said trace 7A having a side edge 7A3, an upper surface 7A1 and a lower surface 7A2, wherein said lower surface 7A2 coupled with said upper surface 41 of insulator 40, a portion of trace 7A accommodated in said blind via 44, in this manner, said trace 7A electrically connected to said ball pad 7B through said blind via 44; a solder mask 80 covering both said upper surface 41 of insulator 40 and a portion of said trace 7A, wherein another portion of said trace 7A being exposed to the atmosphere for external connection; and a plurality of conductive layers 90 which are made of nickel, gold, silver or the like are valuable and expensive, said conductive layers 90 coupled with said upper surface 7B1 of ball pad 7B and the portion of trace 7A exposed to atmosphere (i.e. both said another portion of upper surface 7A1 and said side edge 7A3) respectively; accordingly, (1). The path of transmitting electrical current from the upper surface 41 of insulator 40 to the lower surface 42 of insulator 40 of said conventional PCB 52 is from the upper surface 7A1 of trace 7A through said blind via 44 of insulator 40 to said upper surface 7B1 of ball pad 7B; (2). said solder mask 80 having a thickness Ta which is comprised of a thickness T81 and a thickness T82, wherein said thickness T81 is the distance between the upper surface 7A1 of trace 7A and the upper surface 41 of insulator 40, said thickness T82 is the distance between the upper surface of said solder mask 80 and the upper surface 7A1 of trace 7A, consequently, according to the features of conventional PCB 52 mentioned above, the thickness T52 which is included the thickness T4 of insulator 40 and thickness Ta becomes thicker, in this manner, it is not convenient for said conventional PCB 52 to be used in the electronic industries; (3). said side edge 7A3 of trace 7A also needs to be covered with said conductive layer 90, in this manner, more material needs to be used, then the cost for manufacturing said conventional PCB 52 becomes higher; and (4). Due to there is not any solder mask coupled with the lower surface 42 of insulator 40, then said ball pad 7B can't be employed as a path of transmitting electrical current but be employed as a terminal for external connection (e.g. solder ball) exclusively, in this manner, said ball pad 7B enables not to be layout on said lower surface 42 of insulator 40 freely, moreover, said ball pad 7B is oxidized and damaged easily without solder mask protecting; meanwhile, due to it is difficult for the width “W” of ball pad 7B to be changed, when said ball pad 7B coupled with said lower surface 42 of insulator 40, then it is restricted for said conventional PCB 52 to be used in electronic industry; For example: in case that the volume of solder ball (not shown) is unchanged, and the height of solder ball is needed to be decreased (i.e. said width “W” of ball pad 7B is needed to become wider for reaching the demand mentioned above), nevertheless, due to said width “W” of ball pad 7B can't be changed to become wider for reaching said demand when said ball pad 7B coupled with said lower surface 42 of insulator 40, then the height of solder ball 7B can't be reached to the qualification of electrical device(s) such as PBGA or CSP, in this manner, said conventional PCB 52 enables not to be invited to said electrical devices, as this result, the application of said conventional PCB 52 is restricted.